The present invention relates to an excitation apparatus for controlling an excitation current that is supplied to a generator.
In general, of generators for supplying electric power to either a commercial power line or a large-scale house power generation line, the following type excitation apparatus is called as a self-excitation type excitation apparatus and is widely used: an excitation apparatus in which a current is supplied to a field winding of a generator using electric power outputted from this generator.
In this case, since an excitation current needs to be controlled so as to control a generated voltage, generally speaking, the generated voltage is rectified via a controllable AC/DC converter, and a DC current is supplied to a field winding of a generator.
Recently, since, for example, power transmission distance has increased, it is required for a generator to generate a higher ceiling voltage than that of the conventional generator. The reason why the power transmission distance is increased is given as follows. As capacity of a single generator is increased, it is practically difficult to install a power station in the vicinity of a power consuming place. As apparent from the foregoing fact, large output currents and quick response performance with respect to excitation apparatuses (referred to as a xe2x80x9chigh initial response excitation systemxe2x80x9d) are required.
Currently, static type excitation apparatuses are normally utilized as this high initial response excitation system. This static type excitation apparatus has a superior response characteristic. However, a voltage drop occurring in the primary side of the excitation apparatus is likely to appear as a lowering of output voltage, especially lowering of a ceiling voltage. Since there is a small margin in output voltages, this static type excitation apparatus must be designed by employing extra power to make allowances for this voltage drop, resulting in a waste of performance.
A description will now be made of a conventional static type excitation apparatus.
FIG. 15 is a diagram showing the conventional static type excitation apparatus disclosed in Japanese Patent Application Laid-open No. Sho 64-5400. In this drawing, reference numeral 1 denotes a generator; reference numeral 12 denotes a field winding of the generator 1; reference numeral 2 represents an output line of the generator 1; reference numeral 3 denotes a circuit breaker provided in the generator output line 2; and reference numeral 4 denotes a system bus through which this generator 1 supplies electric power.
Also, reference numeral 5 denotes an excitation transformer connected to the output line 2 of the generator; reference numeral 6 shows a PT for detecting an output voltage of the generator 1; and reference numeral 8 represents an initial excitation circuit used when the generator 1 is first initiated (namely, when the generator is initiated under such a condition that supply of electric power is not received from the system bus 4).
Also, reference numeral 9 denotes a generator voltage adjuster for controlling a thyristor rectifier 10 in such a manner that the output voltage of the PT6 is kept to a predetermined value to control a current of the field winding 12; reference numeral 11 indicates a field switch for protecting a field circuit; and reference numeral 37 denotes a varistor for protecting the field circuit against an overvoltage.
Next, operation of the excitation apparatus of FIG. 5 will now be described with reference to the drawings.
In such a case that while the generator 1 generates the voltage defined in a predetermined range, a load (not shown) connected to the system bus 4 is operated under normal condition and a current defined within the normal range flows, the voltage appearing on the output line 2 of the generator 1 is fed back to the generator voltage adjuster 9 by the PT6. The generator voltage adjuster 9 compares a set voltage value (not shown) with the voltage of the PT6, and controls the current of the field winding 12 with the thyristor rectifier 10 in such a manner that a difference between the set voltage value and the PT6 voltage is reduced.
Assuming now that a short circuit, an earth fault, and the like happen to occur in either the system bus 4 or the load (not shown) connected to this system bus 4, the voltage of the system bus 4 is lowered, and furthermore, the voltage appearing on the output line 2 of the generator 1 is also lowered. As a result, the generator voltage adjuster 9 controls the firing angle of the thyristor rectifier 10 in such a manner that a larger current may flow through the field winding 12. However, since the voltage which is applied via the excitation transformer 5 to the thyristor rectifier 10 is also lowered, even when the firing angle is controlled at the maximum angle, the high voltage which could be originally produced by the thyristor rectifier 10 cannot be produced, and the current is not sufficiently increased.
In other words, the voltage variation occurring on the AC input side of the thyristor rectifier 10 may give no less and, moreover, instant adverse influence to the maximum outputtable voltage of this thyristor rectifier 10. When the AC input voltage is lowered, the maximum outputtable voltage is immediately lowered. As a consequence, in a critical case, the system voltage is and, the necessary excitation voltage cannot not output. Apparently, this drawback may be solved if such an excitation apparatus having considerable extra capacity is designed allowing in advance for the voltage drop in the output line 2. If so, then the resulting excitation apparatus is increased in size and is not economical.
The present invention has been made to solve the above-described drawback of the conventional excitation apparatus, and therefore, provides an excitation apparatus for a generator, capable of applying an excitation voltage and also an excitation current, which are required to output a necessary ceiling voltage even when a voltage of the output generator is lowered due to a failure occurring in a power distribution system.
According to one aspect of the present invention, there is provided an excitation apparatus for a generator, which comprises an AC/AC inverter having a voltage holding function and connected to an output line of an AC generator; and an AC/DC converter controllable with a current and connected to an output side of this AC/AC inverter, a DC output side of this AC/DC converter being connected to a field winding of the AC generator.
According to another aspect of the present invention, there is provided an excitation apparatus for a generator, wherein the AC/AC inverter having the voltage holding function comprises: a capacitor connected to the AC/DC converter and the DC output side of the AC/DC converter; and a DC/AC converter connected to the DC output side of the AC/DC converter.
According to still another aspect of the present invention, there is provided an excitation apparatus for a generator, which comprises an AC/DC converter controllable with a current and connected to an output line of an AC generator by a connection cable having an impedance Z, a DC output side of this AC/DC converter being connected to a field winding of the AC generator, characterized by including an AC/AC inverter having a voltage holding function, the output terminal of which is connected to the connection cable having the impedance Z on the side of the AC/DC converter.
According to a further aspect of the present invention, there is provided an excitation apparatus for a generator, wherein the AC/AC inverter having the voltage holding function comprises: an AC/DC converter connected to another power supply line different from the output line of the AC generator; a DC/AC converter connected to a DC output side of this AC/DC converter; and a capacitor connected to an output side of the AC/DC converter.
According to a still further aspect of the present invention, there is provided an excitation apparatus for a generator, which further comprises instead of the AC/AC inverter having the voltage holding function, a bidirectional AC/DC converter and a capacitor connected to a DC side of this bidirectional AC/DC converter.
According to another aspect of the present invention, there is provided an excitation apparatus for a generator, wherein a series circuit constructed of a semiconductor switch and a capacitor is connected in parallel to the field winding.
According to still another aspect of the present invention, there is provided an excitation apparatus for a generator, which further comprises a charging circuit connected via a charging apparatus to a capacitor.